Ultrapure water production systems for producing ultrapure water from raw water, such as city water, groundwater, or industrial water, basically include a pretreatment system, a primary pure water production system, and a secondary pure water production system. The pretreatment system includes flocculation, floatation, and filtering apparatuses. The primary pure water production system includes two reverse osmosis membrane separators and a mixed-bed ion-exchange apparatus, or an ion-exchange pure water apparatus and a reverse osmosis membrane separator. The secondary pure water production system includes a low-pressure mercury lamp ultraviolet oxidation apparatus, a mixed-bed ion-exchange apparatus, and an ultrafiltration membrane separator.
In recent years, there has been a demand for the production of higher-purity ultrapure water. To this end, it is necessary to sufficiently remove urea in ultrapure water, which prevents a decrease in TOC.
Patent Literatures 1 to 3 disclose that removal of urea from water supplied to an ultrapure water production system can decrease the TOC of ultrapure water.
According to Patent Literature 1 (Japanese Patent Publication 6-63592A (Japanese Patent 3468784)), a biological treatment apparatus installed in a pretreatment system decomposes urea. According to Patent Literature 2 (Japanese Patent Publication 6-233997A (Japanese patent 3227863)), mixed water of water to be treated (industrial water) and recovered semiconductor rinse water is supplied to a biological treatment apparatus installed in a pretreatment system. An organic substance in the recovered semiconductor rinse water serves as a carbon source for a biological treatment reaction and increases the decomposition rate of urea. However, a large number of ammonium ions (NH4+) sometimes contained in the recovered semiconductor rinse water can serve as a nitrogen source like urea and prevent the decomposition of urea. In order to solve this problem, Patent Literature 3 (Japanese Patent Publication 7-313994A (Japanese patent 3417052)) discloses that water to be treated (industrial water) and recovered semiconductor rinse water are separately subjected to biological treatment, are then mixed, and are supplied to a primary pure water production system and a secondary pure water production system.
The addition of a carbon source to water to be treated, as described in Patent Literature 2, can improve the efficiency of decomposition and removal of urea in a biological treatment apparatus but increases the number of bacterial cells grown in the biological treatment apparatus, thus increasing the number of bacterial cells flowing out of the biological treatment apparatus.
Furthermore, as described in Patent Literature 2, use of recovered semiconductor rinse water containing many ammonium ions as a carbon source results in prevention of the decomposition of urea due to the ammonium ions.
Patent Literature 4 (Japanese Patent Publication 9-94585A (Japanese patent 3919259)) discloses that sodium bromide and sodium hypochlorite are added to water to be treated to decompose urea in the water. Paragraphs [0030] and [0039] and FIG. 1 in Patent Literature 4 note that treated water after decomposition and removal of urea with sodium bromide and sodium hypochlorite is supplied to an activated carbon column to decompose and remove sodium hypochlorite.
According to Patent Literature 4, the activated carbon column is to decompose and remove residual sodium hypochlorite ([0039] in Patent Literature 4) and is not to biologically treat sodium bromide and sodium hypochlorite with activated carbon after the addition and decomposition of sodium bromide and sodium hypochlorite.